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United States Patent |
5,728,198
|
Acharya
,   et al.
|
March 17, 1998
|
Process and apparatus for gas purification
Abstract
An air prepurification system which includes vertically oriented adsorption
vessels containing, from top to bottom, a layer of moisture-selective
adsorbent, a first layer of carbon dioxide-selective adsorbent and a
second layer of carbon dioxide-selective adsorbent wherein the particle
size of carbon dioxide-selective adsorbent in the second layer of carbon
dioxide-selective adsorbent is smaller than the particle size of both the
moisture-selective adsorbent and the carbon dioxide-selective adsorbent in
the first layer of carbon dioxide-selective adsorbent. The air
purification system is designed for use in a temperature swing adsorption
process in which air is passed downwardly through the adsorption vessels
during the adsorption step and upwardly through the vessels during the
adsorbent regeneration step.
Inventors:
|
Acharya; Divyanshu R. (Bridgewater, NJ);
Jain; Ravi (Bridgewater, NJ);
Tseng; James K. (Berkeley Heights, NJ)
|
Assignee:
|
The BOC Group. Inc. (New Providence, NJ)
|
Appl. No.:
|
722687 |
Filed:
|
September 30, 1996 |
Current U.S. Class: |
95/114; 95/123; 95/126; 95/139 |
Intern'l Class: |
B01D 053/04; B01D 053/26 |
Field of Search: |
95/98,102,105,114,117-126,130,139
96/130-132
|
References Cited
U.S. Patent Documents
3161488 | Dec., 1964 | Eastwood et al. | 95/118.
|
3237379 | Mar., 1966 | Kant et al. | 95/98.
|
3594984 | Jul., 1971 | Toyama et al. | 95/126.
|
4233038 | Nov., 1980 | Tao | 95/125.
|
4249915 | Feb., 1981 | Sircar et al. | 95/122.
|
4541851 | Sep., 1985 | Bosquain et al. | 96/126.
|
4698072 | Oct., 1987 | Rohde et al. | 95/119.
|
4711645 | Dec., 1987 | Kumar | 95/98.
|
4874525 | Oct., 1989 | Markovs | 95/123.
|
4950311 | Aug., 1990 | White, Jr. | 95/98.
|
4964888 | Oct., 1990 | Miller | 95/95.
|
5110569 | May., 1992 | Jain | 95/123.
|
5198001 | Mar., 1993 | Knebel et al. | 95/126.
|
5202096 | Apr., 1993 | Jain | 95/123.
|
5232474 | Aug., 1993 | Jain | 95/98.
|
5232479 | Aug., 1993 | Poteau et al. | 96/131.
|
5443623 | Aug., 1995 | Jonas et al. | 95/105.
|
5447558 | Sep., 1995 | Acharya | 95/122.
|
5531809 | Jul., 1996 | Golden et al. | 95/105.
|
5593475 | Jan., 1997 | Minh | 95/123.
|
Foreign Patent Documents |
2627327 | Nov., 1977 | DE | 95/118.
|
55-137026 | Oct., 1980 | JP | 95/122.
|
59-004415 | Jun., 1982 | JP.
| |
61-029768 | Jul., 1986 | JP | 95/117.
|
1068150 | Jan., 1984 | SU | 95/119.
|
1380764 | Mar., 1988 | SU | 95/105.
|
2055609 | Mar., 1981 | GB | 95/105.
|
0 449 576 | Mar., 1991 | GB.
| |
Primary Examiner: Spitzer; Robert
Attorney, Agent or Firm: Reap; Coleman R., Pace; Salvatore P.
Claims
What is claimed is:
1. A cyclical process for the separation of a first component of a gas
mixture from a second component of the gas mixture comprising the steps:
(a) passing said gas mixture at superatmospheric pressure downwardly
through an adsorption vessel having at least two layers of particulate
adsorbent, including a first layer of first component-selective adsorbent
and, positioned below said first layer, a second layer of first
component-selective adsorbent, the average particle size of adsorbent in
said first layer being greater than the average particle size of adsorbent
in said second layer, and withdrawing first component-depleted gas from
said vessel at or near its bottom; and
(b) passing a first component-lean purge gas upwardly through said vessel
at a linear velocity between the minimum fluidization velocity of the
adsorbent in the uppermost layer of adsorbent in said vessel and the
minimum fluidization velocity of the adsorbent in said second layer; and
withdrawing first component-enriched gas from said vessel at or near its
top.
2. The process of claim 1, wherein said first layer of adsorbent is the
uppermost layer in said vessel.
3. The process of claim 1, wherein said cyclical process is TSA.
4. The process of claim 3, wherein said gas mixture is air and said first
component is carbon dioxide.
5. The process of claim 4, wherein the adsorbent in said first and second
layers is a zeolite.
6. The process of claim 4, wherein said vessel contains a layer of
moisture-selective adsorbent positioned above said first layer of
adsorbent.
7. The process of claim 6, wherein said moisture selective adsorbent is
silica gel, alumina, zeolite 3A or mixtures of these.
8. The process of claim 7, wherein the adsorbent in said first and second
layers is zeolite 5A, zeolite 13X, calcium-exchanged type X zeolite or
mixtures of these.
9. The process of claim 6 carried out in a plurality of adsorption vessels
operated out of phase such that step (a) is carried out in one adsorption
vessel while step (b) is being carried out in another vessel.
10. The process of claim 9, wherein substantially all of the adsorbent in
said first layer of adsorbent has a particle size in the range of about 2
to about 10 mm and substantially all of the adsorbent is said second layer
of adsorbent has a particle size in the range of about 1 to about 5 mm.
11. The process of claim 10, wherein substantially all of the
moisture-selective adsorbent has a particle size in the range of about 2
to about 12 mm.
12. The process of claim 11, wherein during step (b) said first
component-lean purge gas is passed through said vessel at a linear
velocity in the range of about 0.1 to about 1 meter per second.
13. The process of claim 9, wherein substantially all of the
moisture-selective adsorbent has a particle size in the range of about 5
to about 8 mm, substantially all of the adsorbent in said first layer of
adsorbent has a particle size in the range of about 2 to about 6 mm and
substantially all of the adsorbent is said second layer of adsorbent has a
particle size in the range of about 1 to about 3 mm.
14. The process of claim 1, wherein the adsorbent in said first layer of
first component-selective adsorbent and the adsorbent in the lower layer
of first component-selective adsorbent are the same adsorbent.
15. The process of claim 1, wherein said pressure is in the range of about
2 to about 20 bara.
16. The process of claim 1, wherein the particle size of the adsorbent in
the uppermost layer of adsorbent in said vessel is at least 120% the
particle size of the adsorbent in said second layer.
17. The process of claim 1, wherein said adsorption vessel is a vertical
adsorption vessel.
Description
FIELD OF THE INVENTION
This invention relates to the purification of gases by a cyclic adsorption
process, and more particularly to the removal from a gas stream of a
selected component of the gas stream by temperature swing adsorption (TSA)
in one or more adsorption vessels each containing two or more layers of an
adsorbent that is selective for the selected component, and wherein the
particle size of the adsorbent in one layer is larger than the particle
size of the adsorbent in another layer.
BACKGROUND OF THE INVENTION
When oxygen, nitrogen and argon product gases are produced by fractional
distillation of air at cryogenic temperatures it is necessary to remove
substantially all of the moisture and carbon dioxide from the air feed to
the distillation plant; otherwise these components would freeze in the
heat exchangers and other equipment of the plant, and eventually prevent
the flow of the feed air through the equipment. The removal of water vapor
and carbon dioxide from the feed air is commonly accomplished by passing
the feed air through a TSA-based prepurification unit (PPU) comprising one
or more beds of adsorbent that selectively adsorb these components. As the
demand for oxygen, nitrogen and argon increases, larger air separation
plants are being constructed, and to accommodate the higher capacity of
the larger plants it is necessary to construct larger PPUs. Conventional
vertical PPUs no longer remain economical, and the use of horizontal
vessels becomes necessary. Horizontal vessels require larger bed areas and
create problems with internal gas flow distribution, and they are usually
considered to be more complex to design and operate. There is, therefore,
a great deal of incentive to increase the maximum throughput of
conventional vertical vessels.
The capacity of an adsorption bed of given cross-section and bed depth is
directly proportional to both the velocity of gas passing through the bed
and the effectiveness of the adsorbent in the bed. All other factors being
equal, increasing the velocity of gas flow through the bed will increase
the volume of gas purified. The velocity of gas flow through the bed also
affects the pressure drop across the bed, such that increasing the gas
velocity will cause an increase in the pressure drop across the bed. The
maximum velocity through a fixed bed in downflow mode will depend upon the
crush strength of the adsorbent. The gas velocity through the adsorbent
cannot be increased to the point that the pressure drop is so great that
the adsorbent begins to crumble.
The upward gas velocity through a bed of adsorbent is limited by a second
constraint: the minimum fluidization velocity of the adsorbent. This is
the gas velocity which causes the particles of adsorbent to rise and move
within the bed. Movement of the adsorbent particles in the bed is
undesirable since this causes attrition of the adsorbent, which
dramatically shortens the useful life of the adsorbent. To avoid excessive
adsorbent attrition, a fixed bed adsorption unit is never operated under
conditions which cause fluidization of the adsorbent in the bed.
The effectiveness of the adsorbent is inversely proportional to the
particle size of the adsorbent. Decreasing the particle size of the
adsorbent results in increased effectiveness of the adsorbent. However,
decreasing the particle size of the adsorbent also causes an increase in
the pressure drop across the bed. Decreasing the adsorbent particle size
also lowers the minimum fluidization velocity of the adsorbent.
In conventional adsorption processes the flow of feed gas through the
adsorbent is upwardly during the adsorption step and downwardly during the
bed regeneration. In many instances, since gas flow through a bed during
the adsorption step is considerably greater than gas flow through the bed
during bed regeneration, the minimum fluidization velocity constraint
directly affects the size of the adsorption vessel used in conventional
adsorption processes.
The present invention provides a method of increasing the capacity of an
adsorption unit without increasing the size of the unit, or reducing the
size of the unit while obtaining the same duty. This is accomplished by
using a combination of beds of different sized adsorbent in the adsorption
unit and by reversing the flow of feed gas and bed regeneration purge gas
through the beds, relative to the conventional flow pattern. It is known
to conduct cyclic adsorption processes using a multiple-layer adsorption
system with downflow during the adsorption step and upflow during the
regeneration step wherein the particle size of the adsorbent in the lower
layer is smaller than the particle size of the adsorbent in the upper
layer.
European Patent Application No. 449 576 A1 discloses a conventional upflow
PPU adsorption process in which the adsorbent comprises an upper layer of
fine adsorbent and a lower layer of coarse adsorbent. The disclosure of
this patent is incorporated herein by reference. Bosquain et al., U.S.
Pat. No. 4,541,851 discloses a radial flow adsorption bed containing
adsorbent particles of different sizes. Japanese Kokai Sho 59-4415 (1984)
teaches the use of adsorbent of different particle sizes in the same
vessel. Miller, U.S. Pat. No. 4,964,888 also discloses a PSA process using
adsorbents of different particle sizes.
SUMMARY OF THE INVENTION
The invention comprises a process for purifying a gas stream by adsorption
in an adsorption unit containing an upper layer of coarse particle sized
adsorbent and a lower layer of fine particle sized adsorbent, and wherein
the feed gas flows downwardly through the adsorption unit during the
adsorption step of the cycle and the purge gas flows upwardly through the
unit during the bed regeneration step.
The invention comprises a cyclical process for the separation of a first
component of a gas mixture from a second component of the gas mixture
comprising the steps:
(a) passing the gas mixture downwardly through an adsorption vessel at
superatmospheric pressure, the vessel having at least two layers of first
component-selective particulate adsorbent, including a first layer of
first component-selective adsorbent and, positioned below the first layer,
a second layer of first component-selective adsorbent, the average
particle size of adsorbent in the first layer being greater than the
average particle size of adsorbent in the second layer, and withdrawing
first component-depleted gas from the bottom of the vessel; and
(b) passing a first component-lean purge gas upwardly through said vessel
at a linear velocity less than the minimum fluidization velocity of the
adsorbent in the uppermost layer of adsorbent but not less than the
minimum fluidization velocity of the adsorbent in said second layer of
adsorbent in said vessel; and withdrawing first component-enriched gas
from the top of the vessel.
In a preferred aspect of this embodiment the adsorption process is TSA.
In one aspect of this embodiment the above-mentioned first layer of
adsorbent is the uppermost layer of adsorbent in the vessel.
In another preferred aspect of this embodiment, the gas mixture being
purified is air and the above-noted first component is carbon dioxide. In
this preferred embodiment, the adsorbent in the first and second layers is
a zeolite.
In another aspect of the invention, the vessel in which the adsorption
process is carried out contains a layer of moisture-selective adsorbent
positioned above the above-mentioned first layer of adsorbent. In a
preferred version of this aspect, the moisture selective adsorbent is
silica gel, alumina, zeolite 3A or mixtures of these. In a most preferred
aspect of this embodiment, the adsorbent in the above-mentioned first and
second layers is zeolite 5A, zeolite 13X, calcium-exchanged type X zeolite
or mixtures of these.
In another preferred embodiment the process of the invention is carried out
in a plurality of adsorption vessels operated out of phase such that step
(a), above, is carried out in one adsorption vessel while step (b), above,
is being carried out in another vessel. In a preferred aspect of this
embodiment, substantially all of the adsorbent in the first layer of
adsorbent has a particle size in the range of about 2 to about 10 mm and
substantially all of the adsorbent is the second layer of adsorbent has a
particle size in the range of about 1 to about 5 mm. In a more preferred
aspect of this embodiment, substantially all of the moisture-selective
adsorbent has a particle size in the range of about 3 to about 12 mm.
In a more preferred embodiment of the process of the invention,
substantially all of the moisture-selective adsorbent has a particle size
in the range of about 5 to about 8 mm, substantially all of the adsorbent
in the first layer of adsorbent has a particle size in the range of about
2 to about 6 mm and substantially all of the adsorbent in the second layer
of adsorbent has a particle size in the range of about 1 to about 3 mm.
In a preferred embodiment of the process of the invention, the first
component-lean purge gas is passed upwardly through the vessel during step
(b) at a linear velocity in the range of about 0.1 to about 1 meter per
second.
In a preferred aspect of the process embodiment of the invention, the
adsorbent in the upper layer of first component-selective adsorbent and
the adsorbent in the lower layer of first component-selective adsorbent
are the same adsorbent.
In other preferred embodiments of the invention the gas mixture is passed
downwardly through the vessel at a pressure in the range of about 2 to
about 20 bara; the particle size of the adsorbent in the uppermost layer
of adsorbent in the adsorption vessel is at least 120% the particle size
of the adsorbent in the second layer; and the adsorption vessel is a
vertical adsorption vessel.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a schematic representation of a system in which a first
embodiment of the invention can be carried out; and
FIG. 2 is a schematic representation of a system in which a second
embodiment of the invention can be carried out.
Like reference characters are used in the various figures to designate like
parts of the same equipment units. Only equipment, valves and lines that
are necessary for an understanding of the invention have been included in
the drawing figures.
DETAILED DESCRIPTION OF THE INVENTION
The invention takes advantage of: several gas flow characteristics in
vertical fixed beds of particle adsorbent during cyclic gas adsorption
processes. These include (1) the fact that the volume of purge gas flowing
through an adsorption bed during the bed regeneration step of a cyclic
adsorption process can be much lower than the volume of feed gas flowing
through the bed during the adsorption step; (2) the fact that the downward
velocity of gas through a specific adsorption bed can be much greater than
the minimum bed fluidization velocity of gas flowing upwardly through the
bed; (3) a bed of large particles of a given adsorbent have a higher
minimum fluidization velocity than a bed of small particles of the same
adsorbent; and (4) small particles of a given adsorbent more effectively
adsorb a gas than large particles of the same adsorbent. The invention
also takes advantage of an experimental observation (described later) that
when a layer of large particles is placed on top of a layer of small
particles in a vessel, linear velocity in the upward direction through the
layers can be higher than the minimum fluidization velocity of the small
particles without causing the combined bed to fluidize. The linear
velocity still has to be lower than the minimum fluidization velocity of
the large particles.
The increased capacity of the system of the invention is achieved by
packing fine particle size adsorbent in the lower sections of vertical
adsorption beds of an adsorption system and coarse particle adsorbent in
the upper sections of the beds of the system, and by flowing feed gas
downwardly through the vertical adsorption beds during the adsorption step
of a cyclical adsorption process and flowing purge gas upwardly through
the beds during the bed regeneration step of the process.
The invention can be more fully understood from the following description
taken in connection with the appended drawings. Turning now to FIG. 1 of
the drawings, there is shown therein a two vessel adsorption system
comprised of adsorption vessels A and B. The system is equipped with
piping and valving to permit the adsorption vessels of the system to be
operated in parallel and out of phase, such that the adsorbent in one
vessel is in adsorption service while the adsorbent in the other vessel is
undergoing regeneration. The system can be used to remove any adsorbable
gas impurities from a generally nonadsorbable gas stream, however it will
be described as it is used to remove carbon dioxide and moisture from air.
Vessels A and B are packed with upper layers 2a and 2b, respectively, of
identical water vapor- and carbon dioxide-adsorbent and with lower layers
4a and 4b, respectively, of identical water vapor-and carbon
dioxide-selective adsorbent. As indicated in FIG. 1, the adsorbent in
layers 2a and 2b is larger than that in layers 4a and 4b. Beds 2a and 4a
and beds 2b and 4b may be separated by screens 6a and 6b, if desired.
Screens 6a and 6b are not fastened to the interior walls of vessels A and
B but simply lie on top of layers 4a and 4b.
Feed gas is introduced into the system of FIG. 1 through inlet line 8
which, on its downstream end, is joined to inlet gas manifold 10. Manifold
10 is connected to vessel inlet lines 12a and 12b, which are, in turn,
connected to the inlet ends of vessels A and B, respectively. Manifold 10
contains valves 14a and 14b which control flow between lines 8 and 12a and
between lines 8 and 12b, respectively. The nonadsorbed gas outlet ends of
vessels A and B are connected to vessel outlet lines 16a and 16b,
respectively. The downstream ends of lines 16a and 16b are connected to
product gas manifold 18. Manifold 18 is connected to product gas discharge
line 20. Flow between lines 16a and 20 and between lines 16b and 20 is
controlled by valves 22a and 22b, respectively, each of which are located
in manifold 18. Purge gas inlet line 24 is connected to purge gas inlet
manifold 26 which, in turn, is connected to lines 16a and 16b. Manifold 26
is provided with valves 28a and 28b, which control flow between lines 24
and 16a and between lines 24 and 16b, respectively. Inlet lines 12a and
12b are connected to purged gas outlet manifold 30 which, in turn, is
connected to vent line 32. Manifold 30 is also provided with valves 34a
and 34b, which control flow of gas between lines 12a and 32 and between
lines 12b and 32, respectively.
The process can be used to remove any strongly adsorbed impurities from a
gas stream comprised of a relatively weakly adsorbed gas. However, it will
be described as it applies to the prepurification of air, i.e. the removal
of water vapor and carbon dioxide from the air, by TSA in a two unit
adsorption system operated 180.degree. out of phase, such that one unit is
in adsorption service while the other unit is being regenerated.
In the first half-cycle of the process the adsorbent in unit A is in the
adsorption mode and the adsorbent in unit B is being regenerated, and in
the second half-cycle the adsorbent in unit B is in the adsorption mode
and the adsorbent in unit A is being regenerated. In the first half cycle
valves 14a, 22a, 28b and 34b are open and all other valves are closed.
In practicing the process of the invention in the system illustrated in
FIG. 1, ambient air is introduced into the system via line 8. The air feed
gas stream may pass through a compressor (not shown) wherein the gas is
compressed to a pressure up to, for example, about 10 bara. The compressed
air feed gas stream can then be passed through a heat exchanger (not
shown) wherein it is cooled sufficiently to condense some of the moisture
that may be contained in the gas stream. The cooled gas stream can then be
passed through a water separator (not shown) to remove liquid water
therefrom. These preliminary units are not shown because their use is
optional and depends upon the quality of the inert gas feed stream. The
feed gas stream, now usually at a temperature in the range of about
5.degree. to about 50.degree. C., enters manifold 10.
The compressed feed gas next passes through manifold 10 and line 12a and
into vessel A. The gas passes downwardly through the beds of adsorbent in
vessel A, generally at a linear velocity of about 0.1 to about 1 meters
per second (mps). This velocity can be greater than the minimum
fluidization velocity of the adsorbent in layer 4a; however, since flow of
the gas through vessel A is downwardly and layer 4a is at the bottom of
vessel A, the adsorbent in layer 4a will not be fluidized. As the feed gas
passes through vessel A in the early stage of the half-cycle substantially
all of the moisture and carbon dioxide will be adsorbed in layer 2a. The
mass transfer zone in layer 2a will be relatively large, however, because
of the large size of the adsorbent in layer 2a. As the half-cycle
proceeds, layer 2a will become more and more saturated with water vapor
and carbon dioxide and the adsorption front will approach and enter layer
4a. Eventually layer 2a becomes fully saturated with water vapor and
carbon dioxide. At this point the mass transfer zone moves entirely into
layer 4a. Since the particle size of the adsorbent in layer 4a is smaller
than that of the adsorbent in layer 2a, the adsorbent in layer 4a will be
more efficient than that in layer 2a; accordingly, the mass transfer zone
in layer 4a will be shorter than it was in layer 2a. Thus, the adsorbent
will be more fully utilized than it would be if vessel A contained only
one layer of large sized adsorbent. Furthermore, since the particle size
of the adsorbent in layer 2a is relatively large, the pressure drop in
this layer will be lower than if the entire bed were composed of small
particle size adsorbent. The large particle size of the adsorbent in layer
2a does not adversely affect the efficiency of the process in the final
stage of the half-cycle, since during the final stage layer 2a is in the
equilibrium state.
During this half-cycle of the process the adsorbent in vessel B undergoes
regeneration. This is accomplished by passing a purge gas that contains
very little or no water vapor or carbon dioxide upwardly through vessel B
at a linear velocity in the range of about 0.1 to about 1 mps. This
velocity can be above the minimum fluidization velocity of the adsorbent
in layer 4a but it is not as great as the minimum fluidization velocity of
the adsorbent in layer 2a. Again, fluidization of the adsorbent in layer
4a is avoided because layer 4a is restrained from fluidization by layer
2a.
A suitable gas for use as a purge gas is the prepurified air product gas
from the system or one of the waste gas streams from the downstream air
separation plant. The purge gas is heated by gas heating means (not
shown), generally to a temperature in the range of about 75.degree. to
about 300.degree. C., prior to its introduction into vessel B. The heated
purge gas enters the system through line 24 and passes through line 16b
and into vessel B. As the hot purge gas passes upwardly through the layers
of adsorbent in vessel B, water vapor and carbon dioxide are desorbed from
the adsorbent. The desorbed water vapor and carbon dioxide, together with
the purge gas, pass out of vessel B through lines 12b and 32 and are
vented to the atmosphere.
The purge gas can be passed through vessel B at a greater linear velocity
than would be the case if the position of layers 2b and 4b were reversed.
The mode of operation in this invention permits the use of smaller
diameter adsorbent than must be used in the conventional mode of
operation. There is no danger of reaching the minimum fluidization
velocity of the adsorbent in layer 2b because of its large size. Thus,
because the coarser adsorbent is above the finer adsorbent, when fluid
flow is in the upward direction, smaller diameter adsorbent can be used
than in currently practiced bed regeneration procedures.
When regeneration of the adsorbent in vessel B is finished, the heater is
turned off and cool purge gas that contains very little or no water vapor
and carbon dioxide is passed through the adsorbent in vessel B until the
adsorbent is cooled to the desired extent.
At a predetermined point in the first half-cycle, determined usually when
the adsorption front in layer 4a reaches a certain point in that zone, the
first half-cycle is terminated and the second half cycle is begun. At this
point, valves 14b, 22b, 28a and 34a are opened and all other valves are
closed.
The second half-cycle of the process is identical to the first half cycle
except that the phases conducted in vessels A and B are reversed, such
that, in the second half-cycle, the adsorbent in vessel B is in
purification service and the adsorbent in vessel A is regenerated in the
manner described above.
The system of FIG. 2 is similar to that of FIG. 1 except that desiccant
layers 36a and 36b are positioned in vessels A and B, respectively, above
layers 2a and 2b. As was the case in the FIG. 1 system screens 6 can be
placed between layers 36a and 2a and between 36b and 2b, if desired. The
desiccant in layers 36a and 36b can be any adsorbent which more rapidly
adsorbs water vapor than other components of the gas stream. Typical
desiccants include silica gel, activated alumina, type A zeolite and type
X zeolite. Preferred desiccants include silica gel, activated alumina and
zeolite 3A.
The system of FIG. 2 takes advantage of the fact that water vapor is very
rapidly adsorbed, even on adsorbent which has a very large particle size.
Thus the particle size of the adsorbent in layers 36a and 36b can be as
large as or larger than the particle size of the adsorbent in layers 2a
and 2b. Typically the particle size of the adsorbent in layers 36a and 36b
is in the range of about 3 to about 12 mm, and it is preferably in the
range of about 5 to about 8 mm. The particle size of the adsorbent in
layers 2a, 2b, 4a and 4b are the same as they were in the system of FIG.
1.
It will be appreciated that it is within the scope of the present invention
to utilize conventional equipment to monitor and automatically regulate
the flow of gases within the system so that it can be fully automated to
run continuously in an efficient manner.
The invention is further illustrated by the following example in which,
unless otherwise indicated, parts, percentages and ratios are on a volume
basis.
EXAMPLE 1
Multiple-layer adsorption vessels for removing carbon dioxide from an air
stream based on several flow schemes are designed. The designs are based
on an air feed flow rate of 20,000 standard cubic meters per hour at a
pressure of 6.5 bara and a temperature of 5.degree. C. The air feed is
assumed to contain 350 ppm carbon dioxide and has a relative humidity of
100%. The vessels are designed to operate on the following cycle:
Adsorption-288 min.; Heating-90 min.; Cooling-178 min.; Changeover-20 min.
The flow patterns during the adsorption and regeneration steps for the
cases are as follows: Cases 1 and 2-upflow for adsorption, downflow for
regeneration; Cases 3-5-downflow for adsorption and upflow for
regeneration. Details of the designs are set forth in the Table 1.
Regeneration of the adsorbent in the beds is carried out using dry carbon
dioxide-free nitrogen at near atmospheric pressure at the temperatures and
flow velocities specified in Table 1.
TABLE 1
______________________________________
Case 1 2 3 4 5
______________________________________
No. of Layers
2 2 3 3 3
Ads. Part. Size, mm
3/3 1.5/1.5 1.5/3/6
6/3/1.5
6/3/1.5
Bed ID, m 2.05 2.54 2.54 1.69 1.69
Bed Height, m
1.63 0.92 0.92 1.92 1.92
Ads Vel., m/sec
0.25 0.16 0.16 0.37 0.37
Regen. Vel. m/sec.
0.28 0.18 0.18 0.37 0.50
Reg. Gas Temp, .degree.C.
150 150 150 150 100
Ads .DELTA. P, mbar
54 32 29 62 62
Regen. .DELTA. P, mbar
17 18 15 11 18
% Fluidization.sup.1
70 70 70 77 103
% Fluidization.sup.2
70 70 70 24 34
______________________________________
.sup.1 % of minimum fluidization velocity, smallest particle size
.sup.2 % of minimum fluidization velocity, particles in top bed
In Table 1, where two layers are indicated each layer is a carbon dioxide
adsorbent, and when three layers are indicated the top layer is a
desiccant and the second and bottom layers are carbon dioxide adsorbent.
The adsorbent particle size row states the particle size of the adsorbent
in each layer, with the first number being the particle size of the
adsorbent in the uppermost layer, the second number being the particle
size of the adsorbent in the second layer and the third number (Cases 3-5)
being the size of the adsorbent in the lowermost layer.
To achieve the desired result in Case 1, in which the bed contains two
layers of adsorbent having a particle size of 3 mm, it will be necessary
for the bed to have an internal diameter of 2.05 m and a bed height of
1.63 m. At this height and diameter the maximum allowable upflow velocity
is 0.25 m/sec. In Case 2, in which the bed contains two layers of 1.5 mm
particle size adsorbent, and in Case 3, in which the bed contains three
beds and the size of the adsorbent in the uppermost layer is 1.5 mm, a bed
height of 0.92 m will be necessary, but the bed diameter must be increased
to 2.54 m to avoid a higher approach to fluidization. In Cases 4 and 5, in
which the same layers as in Case 2 are used but in reverse order, a bed
height of 1.92 m will be necessary, and the internal diameter of the bed
will be only 1.69 m. Furthermore, a significant increase in feed velocity
can be tolerated in the system of Case 4 than in any of the other three
cases. This is because the larger particle sized adsorbent sits on top of
the smaller particle sized adsorbent. Since the controlling factor in the
cost of the system is the bed diameter, the cost of the Case 4 system will
be much lower than the cost of the other three systems. Case 5 shows
another advantage of the invention. In case 5 a lower bed regeneration
temperature (100.degree. C.) is used. The regeneration velocity required
to regenerate the bed at 100.degree. C. is 0.5 m/sec. This velocity is
over the minimum velocity of the particles in the lowest bed (103%) but is
well under the minimum fluidization velocity of the particles in the top
bed (34%).
Experiments were carried out using a 100 mm diameter bed with different bed
configurations to determine the fluidization velocities for various
configurations. The results are given below. The particle sizes in Table 2
refer to average particles sizes.
TABLE 2
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Fluidization
Case Bed Configuration Velocity ms/s
______________________________________
1 10", 6 mm AA layer 1.28
2 10", 3 mm 13X layer
0.70
3 10", 1.5 mm 13X layer
0.47
4 2" (top layer) of 6 mm AA
0.82
8" (bottom layer) of 1.5 mm 13X
5 2" (top layer) of 6 mm AA
0.93
6" (middle layer) of 3 mm 13X
2" (bottom layer) of 1.5 mm 13x
______________________________________
As can be seen from Table 2, by putting the large particle adsorbent on top
of the small particle adsorbent, the fluidization velocity for the bed is
higher than the fluidization velocity for the small size adsorbent
(compare case 3 with cases 4 and 5). However, the overall fluidization
velocity is lower than the fluidization velocity of the largest size
particles in the system (compare case 1 with cases 4 and 5).
Although the invention has been described with particular reference to
specific equipment arrangements and to specific experiments, these
features are merely exemplary of the invention and variations are
contemplated. The scope of the invention is limited only by the breadth of
the appended claims.
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